1. Field of the Invention
This invention relates to an improvement in closed type mixing/kneading machines.
2. Description of the Prior Art
A closed type mixing/kneading machine is a batch type mixer suitable for kneading rubber and other plastic materials and is particularly indispensable in the tire manufacturing industry for mastication of rubber (kneading for plasticization) or for carbon master batch kneading or blending vulcanizing chemicals.
Closed type mixers usually employ a pair of double-wing rotors 1 each having a long blade 2 and a short blade 3 as shown in FIG. 1 or a pair of four-wing rotors each having two long wings 2 and two short wings 3 as shown in FIG. 2. A charged material 5 is subjected to a high shearing action and undergoes strong plasticization upon passage through a narrow clearance (tip clearance) between the tip 6 of the rotor and a mixer barrel 4 (as shown in FIGS. 3 through 6) simultaneously with micro-dispersion of additives. On the other hand, the wings of each rotor are twisted in a direction of pushing the material toward the center of the barrel as indicated by arrows in FIGS. 1 and 2, and the two rotors are rotated at a predetermined speed ratio. Due to such rotor construction and the rotational speed ratio between the two rotors, the material is mixed by repeated folding and reversing actions to advance the homogenization of the material and micro-dispersion of additives. Thus, the mixing and kneading actions of the closed type mixer/kneader consist of a combination of shearing and mixing actions.
Upon comparing the double and four-wing rotors, the four-wing rotor, which has twice the number of tips as does the double-wing rotor, has a higher shearing action and is capable of attaining plasticization or micro-dispersion of additives in a shorter time period. On the other hand, the double-wing rotor with a smaller number of tips leaves a greater space in the mixing chamber than the four-wing rotor and is therefore advantageous for smooth movement and homogenization of the mixing material as well as for the micro-dispersion of additives. Namely, the four-wing rotor puts an emphasis on the shearing action while the double-wing rotor emphasizes the mixing action. For instance, the four-wing rotors are suitable for the mastication of natural rubber and carbon master batch kneading, while the double-wing rotors are suitable for the dispersion of vulcanizing agents or other chemicals (pro-kneading).
The above-mentioned kneading characteristics unique to the respective rotor constructions can be an advantage or a detriment depending upon the circumstances. Especially, in the current rubber industry, it is difficult to cope with the hardening or softening of the material due to inclusion of a large quantity of filler or oil by the use of one set of double or four-wing rotors.